In astronomy, lunar orbit (also known as a selenocentric orbit) is the orbit of an object around the Moon. As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by spacecraft around the Moon. The altitude at apoapsis (point farthest from the center of attraction) for a lunar orbit is known as apolune, apocynthion, or aposelene, while the periapsis (point closest to the center of attraction) is known as perilune, pericynthion, or periselene, from names or epithets of the moon goddess. Lunar orbit insertion (LOI) is the adjustment to achieve lunar orbit, as undertaken by Apollo spacecraft for example. Low lunar orbit (LLO) are orbits below altitude. They have a period of about 2 hours. They are of particular interest in exploration of the Moon, but suffer from gravitational perturbation effects that make most unstable, and leave only a few orbital inclinations possible for indefinite frozen orbits, useful for long-term stays in LLO. Robotic spacecraft The Soviet Union sent the first spacecraft to the vicinity of the Moon, the robotic vehicle Luna 1, on January 4, 1959. It passed within of the Moon's surface, but did not achieve lunar orbit. Luna 3, launched on October 4, 1959, was the first robotic spacecraft to complete a circumlunar free return trajectory, still not a lunar orbit, but a figure-8 trajectory which swung around the far side of the Moon and returned to the Earth. This craft provided the first pictures of the far side of the Lunar surface. The Luna 10 became the first spacecraft to actually orbit the Moon in April 1966. It studied micrometeoroid flux, and lunar environment until May 30, 1966. A follow-on mission, Luna 11, was launched on August 24, 1966, and studied lunar gravitational anomalies, radiation and solar wind measurements. The first United States spacecraft to orbit the Moon was Lunar Orbiter 1 on August 14, 1966. The first orbit was an elliptical orbit, with an apolune of and a perilune of . Then the orbit was circularized at around to obtain suitable imagery. Five such spacecraft were launched over a period of thirteen months, all of which successfully mapped the Moon, primarily for the purpose of finding suitable Apollo program landing sites. The most recent was the Lunar Atmosphere and Dust Environment Explorer (LADEE), which became a ballistic impact experiment in 2014. Crewed spacecraft The